This invention is directed to a cathodic electrocoating composition that has improved throwing power.
The coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of a coating composition containing an aqueous emulsion of film forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion until a desired coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from a polyepoxide which has been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like.
The resulting coated article is removed from the bath after a set period of time and is rinsed with deionized water. The coating on the article is cured typically in an oven at a sufficient temperature to produce a crosslinked finish on the article.
Cathodic electrocoating compositions and resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in Jerabek et al U.S. Pat. Nos. 3,922,253 issued Nov. 25, 1975; Wismer et al 4,419,467 issued Dec. 6, 1983; Belanger 4,137,140 issued Jan. 30, 1979 and Wismer et al 4,468,307 issued Aug. 25, 1984.
Throwing power of electrocoating compositions continues to be a problem. Throwing power is the degree to which an electrodeposited film penetrates and coats the surfaces of recessed interior areas of an automobile or truck body. Electrodeposition of coatings follows the force lines of the electrical field that exists between the cathode and anode. These force lines diminish as they penetrate into recessed areas and cease to exist when the recessed area is too deep and a coating will not be deposited into such areas.
As automobile and truck body designs change, there is an increased need for electrocoating compositions with increased throwing power that will penetrate into recessed areas. The improved composition of this invention has increased throwing power along with other desirable characteristics such as solvent, corrosion and chip resistance.